DO YOU know the ultimate purpose of physics? From Aristotle’s four element theory, to Newton’s laws of motion and Einstein’s theory of general relativity, physicists have endeavored to come up with a single theory that can elucidate every phenomenon in the universe. Contemporary physics is divided into two main theories: quantum mechanics and general relativity. However, these theories cannot be a theory of everything. General relativity typically deals with the motion of large objects in fairly large regions of space-time, whereas quantum mechanics generally explain scientific phenomena in subatomic units. What theory, then, might be the next candidate for a theory of everything?

Drawbacks of formal physics

In the natural sciences, it is generally known that four fundamental forces- gravity, electromagnetism, stronger nuclear forces, and weaker nuclear forces-govern the universe. The paradox in formal physics is how to combine the theory of general relativity, which describes the motion of gravity within macroscopic structures, with the theory of quantum mechanics, which accounts for the other three fundamental forces acting in microscopic structures. However, as quantum mechanics cannot completely explain how gravity works in the microscopic world, formal physics cannot account for every material phenomena.

For instance, formal physics meets its limit in the study of black holes. According to general relativity, a black hole has the maximum amount of matter, which means it has extremely heavy gravity. However, a black hole acts upon an extremely small area as well, which indicates that the two theories must be used simultaneously to determine the properties of black holes. Yet, when used together, the mathematical equations for the two theories contradict each other, resulting in answers that do not exist in reality, such as imaginary distances and zero dimensional space.

What is superstring theory?

Interestingly, the superstring theory can resolve this issue, replacing the classical idea of point particles with strings. String theory, the predecessor of superstring theory that was first formulated in late 1960s, has advanced into superstring theory through several decades of intense research and the combined efforts of numerous scientists. According to string theory, the fundamental constituents of reality are tiny strings that vibrate with unique frequencies. Every string, according to the theory, has unique harmonics which determine the individual properties of each different fundamental particle.

The "super" in superstring theory stands for supersymmetry. Supersymmetry is quite similar to the Yang and Yin* concepts of eastern philosophy. Likewise, superstring theory predicts that every known particle in the world should have a partner particle- a so-called supersymmetric partner. For instance, there should be a supersymmetric partner electron for each electron as well as a supersymmetric partner string for each string.

Seeing the world through superstring theory

One of the eccentric features of superstring theory is that an explanation of the “string world” requires more than the three dimensions that we see directly in the world around us. The existence of extra dimensions is an indisputable outcome from the mathematics of superstring theory. However, such extra dimensions are small relative to those that we can directly see, and as a result, they are more difficult to detect.

Furthermore, according to superstring theory, there are two kinds of strings: closed string and open string. Also, there are membranes in the universe that set the boundaries of each dimension. An open string has two ended points which can be stuck on the membranes. On the contrary, a closed string does not have any attachment points so that it can float on every dimension among the membranes. Only by sticking to the three-dimensional membrane can these strings interact with the three-dimensional world. Likewise, if energy is comprised of closed strings that can float on every membrane, then it can escape from its affiliated dimension and leak into extra dimensions.

For example, in the theory, light also consists of an open string that is attached to three-dimensional membranes. As a result, people cannot observe other dimensional objects because every object in the world is seen through the reflection of light.Additionally, as mentioned earlier, four forces dominate the universe. One special trait of the four forces is that gravity is much weaker than the other three forces. Superstring theory can clarify this point by supposing that gravitational force might consist of a closed string. It results in gravity spreading to other dimensions, which makes the gravitational force seemingly weaker compared to the other three forces consisting of open strings.

Endeavors to prove the superstring theory

Edward Witten, who is one of the most renowned theoretical physicists, asserts that the Big Bang could have produced a cosmic string so large that it would still be present in today’s universe. It cannot be not visually detected by telescopes. However, if it exists, super massive cosmic strings will distort the adjacent light, which act as lenses in space. As a result, the gravitational lens by a straight section of a cosmic string would produce two identical, undistorted images of the galaxy. In 2003, a group led by Mikhail Sazbin reported the accidental discovery of two seemingly identical galaxies that appeared very close to each other in the sky, leading to speculation that a cosmic string had been found. However, observation by the Hubble Space Telescope (HST) in 2005 proved these clusters to be different galaxies. A cosmic string produces a similar duplicated image of fluctuation in the cosmic microwave background, which can be detected by radio telescope, such as the HST.

Additionally, although the theory predicts that supersymmetric materials exist, nobody has ever witnessed any of these partner particles. To analyze the compositions of a substance, physicists accelerated two particles to move at the speed of light. These particles then collided into each other. As a result, the particles burst into fragments, which enabled physicists to observe the microscopic structure of substances in the subatomic world. Theorists believe the reason why supersymmetric particles have not been detected is that they are much heavier than known particles. Heavier particles indicate that it takes more energy to generate the particles in a collision. Scientists hope that the new particle collider will have enough energy to start producing these partner particles that the theory predicts. It would be strong evidence that superstring theory is heading in the right direction. Hyun Seung-jun(Prof., Dept. of Physics) said “Supersymmetric particle is indispensable to superstring theory. The next step for the physicists to further develop the theory will be to discover this particle, just as they have discovered the Higgs Boson** in 2013, through the Large Hardon Collider (LHC)”

Moreover, energy can escape from three-dimensions and leak into these extra dimensions under certain conditions. Those conditions might be constructed during high energy collisions in the collider. Through these high energy collisions, physicists will find that there is less energy at the end of the collision. If the proportion of energy loss varies from the microscopic world to the macroscopic world, it could very strongly indicate that the energy has seeped into these extra dimensions. It is because the superstring theory postulates that extra dimensions exist in the microscopic world which energy can leak into. This discovery would be strong evidence that the extra dimension is real and that the string theory is correct.

Hyun said “Actually, superstring theory is very hard to prove experimentally. To observe the string, physicists anticipate the 10^19GV of energy is necessary, while LHC, the current largest collider, makes 10^5GV of energy, which is much lesser. However, superstring theory clarifies the unresolved phenomena in the universe and some of new substances that string theory had anticipated were discovered. Superstring theory would precisely put us at the doorstep of a vast universe of things that we could finally explore.”

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If we can find the equation that governs everything, the way the universe has really worked at every time and place would then be revealed. However, no experiments have been conducted to prove the theory. Fortunately, formal physics theories, such as Einstein's theory of general relativity and Maxwell's equation, are on a similar track with superstring theory. Two theories started with a hypothesis and thereafter were proven through experiments. Perhaps they might hold the clue to the master equation of the universe.

*Yang and Yin: Two substances in eastern philosophy that describe how opposite forces are actually interconnected in the natural world, and how they give rise to each other as they relate with one another

**Higgs Boson: An elementary particle in the Standard Model of particle physics that is anticipated to generate the mass of particles